Quantum computer hardware with reflectionless filters for thermalizing radio frequency signals

ABSTRACT

A quantum computer hardware apparatus may include a first stage, which is connected to one or more signal generators, and a second stage adapted to be cooled down at a lower temperature than the first stage. Superconducting qubits are arranged in the second stage. The signal generators are configured, each, to generate radio frequency (RF) signals to drive the qubits, in operation. The apparatus may further include an intermediate stage between the first stage and the second stage, wherein the intermediate stage comprises one or more coolable filters, the latter configured for thermalizing RF signals from the signal generators. Related methods for thermalizing radio frequency signals in a quantum computer hardware apparatus are also disclosed.

BACKGROUND

The present disclosure relates in general to the field of quantumprocessing hardware apparatuses comprising superconducting qubit drivenby radio frequency signals and, in particular, to techniques forthermalizing radio frequency signals in such apparatuses.

Recent advances in quantum computing are making such a technology evermore relevant to industrial applications. Quantum computing makes directuse of quantum-mechanical phenomena, such as superposition andentanglement to perform operations on entangled quantum bits (qubits),i.e., information stored in quantum states. Superconducting circuits arerelatively easy to manufacture with current technologies and are thuscandidates to further scale quantum information technologies. Today, itcan be envisioned that in the near term a small quantum computer, basedon a couple of hundreds of superconducting qubits with error mitigationor limited error correction, will be able to simulate quantum systemsintractable to conventional computers.

Quantum computing devices are known, which are based on superconductingqubits of the transmon type. Such qubits are controlled by radiofrequency (RF) technology. Such qubits need be operated at a temperatureof a few mK only. RF signals are fed into the cryostat with coax cablesusing attenuators placed on intermediate temperature platforms tothermalize the signals for each of the upward and downward path. Theattenuators are cooled to the temperatures of their respectiveplatforms. In total, approximately 60 to 90 dB of attenuation istypically ensured between the signal generator and the qubits, thanks tosuch attenuators.

SUMMARY

According to a first aspect, the present invention is embodied as aquantum computer hardware apparatus. The apparatus includes a firststage, which is connected to one or more signal generators, as well as asecond stage adapted to be cooled down at a lower temperature than thefirst stage. Superconducting qubits are arranged in the second stage.The signal generators are configured, each, to generate radio frequency(RF) signals to drive the qubits, in operation. The apparatus furtherincludes an intermediate stage between the first stage and the secondstage, wherein the intermediate stage comprises one or more coolablefilters, the latter configured for thermalizing RF signals from thesignal generators.

According to another but related aspect, the invention is embodied as amethod for thermalizing radio frequency signals in a quantum computerhardware apparatus. Consistently with the above apparatus, the methodinvolves generating RF signals conveyed through a first stage of theapparatus to drive superconducting qubits arranged in a second stage ofthis apparatus, where the second stage is cooled down at a lowertemperature than the first stage. RF signals generated by the signalgenerators are thermalized at an intermediate stage between the firststage and the second stage, via one or more cooled filters arranged inthis intermediate stage.

A concept underlying an embodiment of this invention relies on cooledfilters (instead of attenuators) for thermalizing RF signals from thesignal generators and thereby reduce both the signal energy dissipatedin the second stage and power needed by the signal generators togenerate the RF signals.

Apparatuses and methods embodying the present invention will now bedescribed, by way of non-limiting examples, and in reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, which together with the detailed descriptionbelow are incorporated in and form part of the present specification,serve to further illustrate various embodiments and to explain variousprinciples and advantages all in accordance with the present disclosure,in which:

FIGS. 1A and 1B are block diagrams schematically illustrating selectedcomponents of a quantum computer hardware apparatus relying on signalattenuators to thermalize radio frequency signals in this apparatus.FIG. 1A shows a high-level, schematic representation, while FIG. 1Baddresses a more detailed solution (FIG. 1B is not prior art); and

FIGS. 2A and 2B show other block diagrams schematically illustratingselected components of an apparatus including coolable filters forthermalizing radio frequency signals from the signal generators, asinvolved in embodiments. FIG. 2A shows a high-level, schematicrepresentation, while FIG. 2B is a more detailed depiction; and

FIGS. 3A and 3B show curves illustrating how selective filters withdynamically adjustable frequencies can be used to adjust the allowedfrequencies for multiplexed applications, as in embodiments. FIG. 3Agenerally illustrates transmission characteristics of a dynamicallyadjustable bandpass filter, while FIG. 3B shows filter characteristics,wherein one frequency range of the signal is more attenuated than theneighboring frequency range.

The accompanying drawings show simplified representations of devices orparts thereof, as involved in embodiments.

DETAILED DESCRIPTION

Attenuators 11 a-41 a, 12 a-42 a are used in quantum informationprocessing apparatuses 1 a with several successive platforms (stages) 10a-50 a, as in FIGS. 1A, 1B. Once cooled to the platform temperature,such attenuators, allow radio frequency (RF) signals (as conveyed alonglines denoted arrows in FIGS. 1A-2B) to be thermalized. Now, while sucha solution reduces the noise, it also substantially reduces theintensity of the useful signal. This can become problematic where alarge number of qubits 55 a are contemplated, especially when asubstantial part of the control electronics is placed in the vicinity ofthe qubit platform, e.g., on a 3-4 K platform. I.e., for a large numberof qubits, the power in the attenuator adds up and if the signal isgenerated close to the 3-4 K platform, then one would prefer to generatelow intensity signals due to the power needed to generate them.

There, it would be advantageous to reduce the number of electricalconnections to the platform at room temperature and improve the delayand real-time behavior of the qubits 55 a (especially on the feedbackpath). Now, an attenuation of 40 to 50 dB is typically needed between anintermediate platform 20 a-40 a (e.g., the 3-4 K platform) and the qubitplatform 50 a, for thermalization purposes. This, however, has adverseconsequences on the drive signals as higher signal levels are needed.Therefore, the present inventors have designed alternative solutions tothermalize RF signals, as now discussed detail.

In reference to FIGS. 2A and 2B, an aspect of the invention is firstdescribed, which concerns a quantum computer hardware apparatus 1.

As usual, this apparatus includes a first stage, which is connected toone or more signal generators (not shown). The apparatus furtherincludes a second stage 50, which is adapted to be cooled down at alower temperature than the first stage.

Superconducting qubits 55 are arranged in the second stage 50. Note,numeral reference 55 may refer to several components and several typesof such components, in embodiments as described below. The signalgenerators are configured, each, to generate RF signals to drive thequbits 55, in operation. While any RF signal may a priori be used (i.e.,signals within 3 10⁵ to 3 10¹¹ Hz) such signals can be microwave signals(i.e., signals whose frequencies are between 3 10⁸ and 3 10¹¹ Hz) andalso can be in the frequency range extending from 4 to 8 GHz.

Such superconducting qubits can be of the transmon type. The apparatus 1may further comprise additional RF-controlled components, such as(tunable) couplers (e.g., frequency-tunable coupling elements). That is,qubits 55 may possibly be connected to one or more tunable couplers(i.e., couplers that contain non-linear elements for its frequency to betunable), e.g., to allow transitions between states of the qubits to beparametrically driven, by modulating the tunable coupler energy. Forexample, two-qubit gates are known, which are implemented with transmon(fixed-frequency) qubits, where the qubits are coupled via such afrequency-tunable coupler. The latter is typically not meant to be usedas a computational qubit. However, a tunable coupler can practically beembodied as a superconducting qubit. In variants, fixed-frequencycouplers may be used.

Such couplers are assumed to form part of the block 55 in FIGS. 2A, 2B.In each of FIGS. 2A and 2B, the elements in the block 55 may likelyinclude multiple qubits and couplers between them, as well otherelements. As said, the couplers may have a fixed frequency or maycontain non-linear elements to make them tunable. One method is to useJosephson junctions as non-linear elements and to tune them using anexternal magnetic field. In variants, a capacitive coupling may berelied upon. Other couplers may be used for different purposes. Forexample, in the embodiment depicted in FIG. 2B, a DC drive is involvedon the RHS, downward path to the for the Josephson parametric amplifier(JPA), e.g., a low noise JPA, which typically involves a tunableadjustment scheme for its center frequency, used to adjust and optimizethe amplifier on the qubit frequencies.

As illustrated in FIG. 2A and FIG. 2B, the apparatus 1 additionallyincludes one or more intermediate stages 10-40, each arranged betweenthe first stage and the second stage 50. Note, the stage referred to asthe “first stage” above is an upstream stage, which is not necessarilyat room temperature (RT). Rather, this “first stage” may already be acooled stage, such as the 4 K platform 20 in FIG. 2. The second stage 50is the coldest stage. The successive stages 10-50 (from the top stage 10all the way down to the second stage 50) form a cryogenic signal line.Additional stages (not shown) may possibly be involved “above” stage 10.

Here, each intermediate stage 10-40 comprises one or more coolablefilters 11-41, also referred to as cooled filters (once cooled, inoperation of the apparatus 1) or as cold filters. In the presentcontext, such filters are configured for thermalizing RF signals asobtained from the signal generators. Moreover, additional coolablefilters 11, 51 will likely be arranged in the first and/or the secondstage 50 as well. For example, the second stage 50 may advantageouslyinclude an Eccosorb filter (trade name), labeled as “ESF” filter 51 inFIG. 2B. Similarly, the very first stage (e.g., stage 10) may include acoolable filter 11. I.e., each of the stages involved may include one ormore coolable filters.

Such filters 11-51 should be distinguished from mere attenuators. Aconcept underlying an embodiment of this invention is to rely on cooledfilters instead of attenuators for thermalizing RF signals from thesignal generators. As it may be realized, this makes it possible tooptimize and reduce both a signal energy dissipated in the apparatus 1and the power needed by the signal generators to generate the RFsignals. This further allows the noise bands to be blocked down tolevels determined by the thermal energy of the respective stages.Indeed, thanks to filters 11-51 as used herein, a smaller intensitysignal is needed and therefore less power is dissipated and thus lesspower is needed in the generator. On the contrary, prior art solutionstypically rely on attenuators 11 a-51 a, 12 a-52 a, as assumed inFIG. 1. Note, however, that the specific arrangement of platforms andelectronics shown in FIG. 1B is not necessarily prior art.

In more detail, less driving signal amplitude is needed asthermalization is at least partially achieved by using filters insteadof attenuators. This translates in less dissipated signal power in thecryostat, it being reminded that with a large number of qubits andsignal lines, the dissipated power adds up.

The present solution is particularly interesting if the signal isinitially generated on a cooled platform, which is, e.g., at atemperature between 2 and 6 K, and more likely between 3 and 5 K, thisdepending on the thermal load and other factors that locally impact thetemperature. With respect to power dissipation, especially below 3 Kwhere cooling gets inefficient, there is no need to generate a largesignal amplitude in the present case thanks to the proposed filterconfiguration. As less signal gets dissipated though the filters 11-51,less power is needed in the first place, which also simplifies thegeneration of high-fidelity signals. Moreover, the present solutionmakes it possible to reduce the required signal level in such a way thatit becomes compatible with standard low power CMOS, even at roomtemperature. I.e., said intermediate stage may possibly compriseCMOS-fabricated components, the latter including said one or morecoolable filters.

Thus, by reducing power dissipation, embodiments of the inventioneliminate the need for RF amplifiers, enable implementations in CMOS,and thereby opens the door to integration of large systems.

All this is now described in detail, in reference to particularembodiments of the invention. To start with, and as illustrated in FIGS.2A, 2B, the apparatus 1 in one embodiment comprises several intermediatestages 10-40 arranged between the first stage and the second stage 50.Thus, the first stage, the intermediate stages 10-40, and the secondstage 50 form a series (i.e., a chain) of successive stages, whereineach intermediate stage is adapted to be cooled down at a lowertemperature than any previous stage (on top therefrom in FIGS. 2A, 2B)of the series. As further seen in FIGS. 2A, 2B, each intermediate stagecomprises one or more coolable filters 11-41, 12-42, wherein each of thefilters is configured for thermalizing RF signals throughout theintermediate stages.

As evoked earlier, the stage considered as the “first stage” is notnecessarily at RT (and thus is not necessarily the topmost stage in thehierarchy instituted by FIGS. 2A, 2B). Plus, any of the platforms 10-30shown in FIGS. 2A, 2B may here be considered as the “first stage”,inasmuch as there is at least one intermediate platform in between.Thus, the first stage may by an intermediate stage 10-30 of thehierarchy shown in FIGS. 2A, 2B, which stage may be adapted to be cooledat an already low temperature, e.g., between 2 and 6 Kelvin, inoperation of the apparatus. The first stage may for example be cooleddown to 4 K, as assumed for the platform 20 in FIGS. 2A, 2B. The presentsolution is especially interesting where RF signals are generated onsuch a platform, since there are still approximately 40 dB ofthermalization needed between this platform and the qubit's.

The coolable filters 11-51, 12-52 are in one embodiment designed asreflectionless filters, which absorb signal in their stopband, insteadof rejecting signals by reflecting them back outside of the passband.This way, the rejected signals do not combine. As the skilled person mayappreciate, a cold reflectionless filter may be designed so as to onlyemit a noise signal level according to its temperature (imposed by therespective cooling stage), while attenuating and dissipating only asmall amount of the useful signal desired. On the contrary, a classicalLC filter transmits (substantially) all the signals inside its passbandbut reflects (substantially) all the signals in the stopband(s). As aresult, the filter is matched (i.e., has a small S11 parameter) in thepassband but has a high S11 parameter in the stopband. On the contrary,a reflectionless filter provides a match to all frequencies. I.e., thesignal is fed through inside the passband, while being absorbed in thestopband. That is, all the signals are allowed in the passband, whileall the signals are absorbed in the stopband (i.e., a lot more than itis reflected). Advantageously here, the absorption in the stopband isnot an issue in terms of power consumption, since the signal absorbedwill merely be noise in the present context. In addition, areflectionless filter makes a clean thermal emitter in the stopband,which is desirable in applications as contemplated herein.Reflectionless filters prevent multiple reflections to combine, as notedabove.

Advantages of reflectionless filters in terms of power consumption openthe door to CMOS implementations. On the contrary, using attenuators forthermalization purpose typically require between 10 and 13 dBm of inputsignal power (corresponding to ˜1 V rms in a 50 Ohm system), which isvery difficult to achieve with CMOS components, even at 300 K, due tothe low maximum supply voltage allowed by this technology (whichcorresponds to ˜0.7 V to 1 V in practice, depending on the actualimplementation).

For example, in embodiments, each of above coolable filters (or at leastsome of them) is configured as a reflectionless bandpass filter, thusallowing a signal inside its passband, while absorbing signal (noise) inits stopband, instead of reflecting it. Note, such bandpass filters canbe designed for multiple signals having different attenuations.

Each of above coolable filters (or at least some of them) may possiblybe configured so as to have dynamically adjustable bandpasscharacteristics. E.g., the passband frequency of such filters may becontrolled by means of control devices such as switches, transistors orvaractors, for example. Thus, such filters can be configured asselective filters, whose bandpass is adjusted to the qubit frequencyand/or the readout frequency. Multiple readout frequencies ofmultiplexed qubits can accordingly be conveyed over a single line (asassumed in FIG. 2B). Similarly, multiplexed qubit drives can beachieved, if necessary.

As said, the apparatus 1 may possibly include additional RF-controlledcomponents, such as couplers (e.g., frequency-tunable couplers, as partof block 55). Thus, cooled filters may be used for thermalizing thequbit control signals and/or coupler signals. That is, one may want touse filters on the path to the coupler(s) connected between the qubits,as in embodiments. Thus, additional coolable filters 12-42 can bearranged in the intermediate stages, as illustrated in FIGS. 2A and 2B.E.g., a first set of coolable filters 11-51 may be arranged on thedownward path to the qubits, while a second set of coolable filters12-42 may be arranged on the path to the directional coupler 52. Eachfilter of the second set is configured for thermalizing RF signalstransmitted to this coupler 52. More generally, cooled filters may beused for thermalizing the qubit control signals, the coupler signals,and/or the readout signals (not shown).

As further illustrated in FIGS. 2A, 2B, each filter of the second setmay be configured as a narrowband reflectionless bandpass filter. Eventhough the need for filters on the path to the couplers or on the returnpath is less stringent than on the path to the qubits, such filters canbe implemented as reflectionless filters, to mitigate adverse backactionto the qubits, which would else potentially lead to decoherence.

In fact, cold filters 11-51, 12-42 may possibly replace all attenuatorsmeant to thermalize the RF signals, as assumed in FIG. 2 (compare FIGS.1B and 2B). Thus, in embodiments, none of the intermediate stagecomprises thermalizing attenuators (i.e., attenuators configured toattenuate a RF signal from the signal generators, the qubits, or thetunable couplers). Still, depending on the implementation chosen, RFsignals may still need be slightly attenuated in the passband of thefilters too, though less than in their stopbands, as such filters willlikely attenuate the signal as possible in their stopbands and, this,down to the temperature of the cooling stage. Thus, in variants,attenuators may still be needed, in addition to the present filters.

According to another aspect, the present invention can be implemented asa method for thermalizing RF signals in a quantum computer hardwareapparatus such as described above.

This method revolves around generating RF signals (at or upstream afirst stage of the apparatus), in order to drive superconducting qubitsarranged in a second stage, where the latter is cooled down at a lowertemperature than the first stage. Meanwhile, the RF signals generated bythe signal generators are thermalized via one or more cooled filters11-41, at one or more intermediate stages 10-40 between the first stageand the second stage 50.

Moreover, a similar filter arrangement may be implemented on the path tothe coupler 52 and/or on the return path (from the qubits), as discussedearlier. I.e., RF signals transmitted to the coupler 52 and/or returnedfrom the qubits in one embodiment are thermalized via additional set(s)of cooled filters 12-42 arranged at the level of said intermediatestage(s).

The above embodiments have been succinctly described in reference to theaccompanying drawings and may accommodate a number of variants. Severalcombinations of the above features may be contemplated. Examples aregiven below.

FIG. 2 B shows a more detailed version of the system depicted in FIG.2A. The downward signal path labeled “to Qbit” involves reflectionlessbandpass filters 11, 21, 31, 41, 51 cooled to respective platformtemperatures. Each of these bandpass filters ensures little attenuationfor the desired control signal traveling from the generator(s) towardsthe qubits. On the other hand, the out-of-band noise is absorbed in thereflectionless bandpass filters 11-51. This way a thermalization of thesignal is achieved with a controlled signal attenuation. For example, ina typical implementation, the filter 41 will approximately provide 10 to20 dB of attenuation, to reduce the signal to the very small amplitudeneeded to excite the qubits. On the 20 mK platform the signal is passedthrough a further thermalizing bandpass filter including an Eccosorbfilter (as part of the ESF component 51), whose role is to remove anyresidual, high-frequency radiation (typically in the infrared frequencyrange). The next block 55 (labeled “Q”) denotes multiple deviceelements, which may include one or more qubits, couplers, read-outresonators, and other components, as noted earlier. The output side ofblock 55 is first connected to an isolator, i.e., a passive device whichallows RF signal to propagate in only one direction. This isolatorallows signal to propagate in the desired direction, while blockingnoise coming from the other direction. The block 52 denotes adirectional coupler, i.e., another passive microwave device combiningthe pump signal for the JPA and the qubit readout signal. The pathlabeled “JPA pump” provides the pump signal for the JPA, from, e.g.,room temperature, down to the mK level. A series of reflectionlessfilters 12, 22, 32, 42 are again relied on, each thermally attached to arespective temperature platform. Their function is similar to that ofthe filters used in the first path, although here more narrowbandfilters can be used as the pump signal is at a defined frequency. Onlythis pump signal is needed to pass the filters. The pump signal and thequbit readout signal are leaving the coupler 52 and are fed into acirculator, i.e., a passive microwave device (here having threeterminals). The signal can propagate in only one direction (the counterclockwise direction in FIG. 2B). Such a circulator is known per se, itrelies on magnetism as used, e.g., in a radar to separate the RX and TXpath connected to one antenna. In the present system at the first outputof the circulator a high pass filter is connected and afterwards is aJPA. The amplified signal is reflected, passes again trough thecirculator and exits at its third output. Through two isolators and alow pass filter the signal is fed into a traditional, electrical, lownoise amplifier, which further amplifies the readout signal. The lastpath, labeled “JPA (DC)”, supplies the necessary DC bias for the JPA,again trough some attenuator, feed through for thermalization.Typically, also in this path some ESF filter is used to keep any thermalnoise out of the system.

Advantages achieved in embodiments of the present invention may include:

-   -   Less driving signal amplitude is needed if thermalization is at        least partially achieved thanks to the cooled filters (e.g.,        selective filters) instead of attenuators;    -   Using cooled filters allow less dissipated signal power in the        cryostat, it being reminded that dissipated power would        critically add up with a large number of qubits and signal        lines;    -   The present solutions are particularly advantageous if RF        signals are generated on a 3-4 K platform; and    -   The required signal level can be reduced to an extent such that        standard CMOS hardware become capable to generate the required        signal levels. In variants, a GaAs-based technology can be        contemplated, amongst others.        Some embodiments may not have these potential advantages and        these potential advantages are not necessarily required of all        embodiments.

In some embodiment, instead of attempting to improve the qubits, presentembodiments aim at improving the required drive power to a level whereit can be supplied directly with standard technology (e.g., CMOS),without applying expensive RF amplification, which, in turn, opens thedoor to direct implementation in such standard technology andintegration into large systems.

While the present invention has been described with reference to alimited number of embodiments, variants and the accompanying drawings,it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted without departing fromthe scope of the present invention. In particular, a feature(device-like or method-like) recited in a given embodiment, variant orshown in a drawing may be combined with or replace another feature inanother embodiment, variant or drawing, without departing from the scopeof the present invention. Various combinations of the features describedin respect of any of the above embodiments or variants may accordinglybe contemplated, that remain within the scope of the appended claims. Inaddition, many minor modifications may be made to adapt a particularsituation or material to the teachings of the present invention withoutdeparting from its scope. Therefore, it is intended that the presentinvention not be limited to the particular embodiments disclosed, butthat the present invention will include all embodiments falling withinthe scope of the appended claims. In addition, many other variants thanexplicitly touched above can be contemplated. For example, in FIG. 2B,the “low pass” behind the ‘superconductive coaxial cable and the “highpass” on the LHS of the JPA may involve a band pass filter, which maynot necessarily be a thermalizing filter. Also, in the block 51 (“ESF+BPfilter”), the BP filter may likely be a thermalizing filter.

What is claimed is:
 1. A quantum computer hardware apparatus,comprising: a first stage connected to a signal generator; a secondstage with superconducting qubits arranged therein, the second stageadapted to be cooled down at a lower temperature than the first stage,wherein the signal generator is configured to generate radio frequencysignals to drive the qubits; and an intermediate stage between the firststage and the second stage, wherein the intermediate stage comprises atleast a coolable filter, the coolable filter configured for thermalizingradio frequency signals from the signal generator, wherein the apparatuscomprises several intermediate stages, the intermediate stages includingat least said intermediate stage, wherein the several intermediatestages are arranged between the first stage and the second stage, so asfor the first stage, the intermediate stages, and the second stage toform a series of stages, and each of the intermediate stages is adaptedto be cooled down at a lower temperature than any previous stage in theseries, and comprises at least a coolable filter, the coolable filterconfigured for thermalizing said radio frequency signals throughout theintermediate stages.
 2. The quantum computer hardware apparatusaccording to claim 1, wherein the first stage is adapted to be cooled ata temperature between 2 and 6 Kelvin, in operation of the apparatus. 3.The quantum computer hardware apparatus according to claim 1, whereinthe coolable filter is configured as a reflectionless bandpass filter,allowing a signal inside a passband thereof while absorbing signal in astopband thereof.
 4. The quantum computer hardware apparatus accordingto claim 3, wherein the coolable filter is configured so as to havedynamically adjustable bandpass characteristics.
 5. The quantum computerhardware apparatus according to claim 1, wherein the coolable filterbelongs to a first set of coolable filters, the second stage furtherincludes at least one coupler connected to the superconducting qubits,and said intermediate stage further includes a second set of at leastone coolable filter, each configured for thermalizing radio frequencysignals transmitted to said at least one coupler.
 6. The quantumcomputer hardware apparatus according to claim 5, wherein each of the atleast one coolable filter of the second set is configured as anarrowband, reflectionless bandpass filter.
 7. The quantum computerhardware apparatus according to claim 1, wherein said intermediate stagedoes not include any attenuator configured to attenuate a radiofrequency signal from said signal generator.
 8. The quantum computerhardware apparatus according to claim 1, wherein said intermediate stagecomprises CMOS-fabricated components, the CMOS-fabricated componentsincluding said coolable filter.
 9. The quantum computer hardwareapparatus according to claim 1, wherein each of said superconductingqubits is of the transmon type.
 10. The quantum computer hardwareapparatus according to claim 1, wherein the second stage furtherincludes at least one frequency-tunable coupling element, each coupledto two or more of the qubits.
 11. A method for thermalizing radiofrequency signals in a quantum computer hardware apparatus, the methodcomprising: generating radio frequency signals conveyed through a firststage of the apparatus to drive superconducting qubits arranged in asecond stage, while cooling down the second stage at a lower temperaturethan the first stage; and at an intermediate stage between the firststage and the second stage, thermalizing the radio frequency signalsgenerated by signal generators via a cooled filter arranged in saidintermediate stage, wherein the apparatus comprises several intermediatestages, the several intermediate stages including at least saidintermediate stage, wherein the several intermediate stages are arrangedbetween the first stage and the second stage, so as for the first stage,the intermediate stages, and the second stage to form a series ofstages, and wherein each of the several intermediate stages comprises acooled filter, and the method comprises, while generating said radiofrequency signals, cooling down each of the intermediate stages at alower temperature than any previous stage in the series, andthermalizing said radio frequency signals throughout the intermediatestages via the cooled filter thereof.
 12. The method according to claim11, wherein the method further comprises cooling down the first stage ata temperature between 2 and 6 Kelvin, while cooling down the secondstage at a lower temperature than the first stage.
 13. The methodaccording to claim 11, wherein the cooled filter is configured as areflectionless bandpass filter, said radio frequency signals beingthermalized by allowing a signal inside a passband of said cooled filterwhile absorbing a signal in a stopband of said filter.
 14. The methodaccording to claim 13, wherein the method further comprises dynamicallyadjusting bandpass characteristics of the cooled filter arranged in theintermediate stage.
 15. The method according to claim 11, wherein saidcooled filter belongs to a first set of cooled filters, the second stagefurther includes at least one coupler connected to the superconductingqubits, and said intermediate stage further includes a second set of atleast one cooled filter, the method further including, at saidintermediate stage, thermalizing radio frequency signals transmitted tosaid at least one coupler via the second set of at least one cooledfilter.
 16. The method according to claim 15, wherein each of the atleast one cooled filter of the second set is configured as a narrowbandreflectionless bandpass filter.
 17. A quantum computer hardwareapparatus, comprising: a first stage connected to a signal generator; asecond stage with superconducting qubits arranged therein, the secondstage adapted to be cooled down at a lower temperature than the firststage, wherein the signal generator is configured to generate radiofrequency signals to drive the qubits; and an intermediate stage betweenthe first stage and the second stage, wherein the intermediate stagecomprises at least a coolable filter, the coolable filter configured forthermalizing radio frequency signals from the signal generator, whereinthe coolable filter is configured as a reflectionless bandpass filter,allowing a signal inside a passband thereof while absorbing signal in astopband thereof.
 18. The quantum computer hardware apparatus of claim17, wherein the coolable filter is configured so as to have dynamicallyadjustable bandpass characteristics.